President Donald Trump on Monday pledged a new strategy in Afghanistan, while refusing to divulge troop numbers, as he said the office of the president had changed his instincts to pull out of the nearly 16-year-long war.

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In a sober yet vague address, Trump laid out his rationale for continuing the foreign entanglement, despite having pledged during the campaign to end America’s engagement in wars without clear exits.

“My original instinct was to pull out, and historically I like following my instincts, but all of my life I heard that decisions are much different when you sit behind the desk in the Oval Office,” Trump said in a prime-time address to the nation from the Fort Myer military base in Arlington, Virginia. “However, our commitment is not unlimited, and our support is not a blank check. The American people expect to see real reforms and real results.”

Trump repeatedly blasted President Barack Obama for too hastily withdrawing from Iraq and for his preference for timetables in determining troop drawdowns. He promised a “dramatically” different approach to the war, with future troop levels based on conditions in the country, rather than any preset time frame.

Trump spoke in broad terms about the need for American allies, including India, to contribute to Afghanistan’s economic development, and was quick to stress the military, rather than political, goals of the war.

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“We must stop the resurgence of safe havens that allow terrorists to threaten America,” Trump said. “We are not nation-building again. We are killing terrorists.”

“Our troops will fight to win” the president said. “We will fight to win. From now on, victory will have a clear definition.”

But no such clear definition was forthcoming. Trump promised “principled realism” and “strategically applied force,” and issued warnings both to militants and American allies in the region.

The announcement comes after the president and senior advisers wrestled with the decision of how to proceed for months. Trump has previously expressed deep skepticism of American military involvement abroad, but officials from Defense Secretary James Mattis to national security adviser H.R. McMaster advocated for an increased presence in Afghanistan. One of the loudest voices against such a surge, White House chief strategist Steve Bannon, was fired on Friday.

Breitbart News, the right-wing site Bannon worked at before joining the White House and that he returned to Friday, hit Trump after the speech for what it called his “flip-flop.” But the address generally received positive reviews, with House Speaker Paul Ryan praising Trump for offering a “more comprehensive strategy.” Sen. John McCain (R-Ariz.), the chairman of the Armed Forces Committee, also praised Trump for “moving us well beyond the prior administration's failed strategy of merely postponing defeat.”

Democrats expressed skepticism about the address, with House Minority Leader Nancy Pelosi saying that “the American people need to know more about the president's plans and conditions.” She criticized the speech as “low on details.”

More than 2,400 Americans have died in the war since the initial invasion in October 2001, along with more than 1,000 coalition troops and over 30,000 soldiers and police officers with the U.S.-allied Afghan government forces. More than 31,000 Afghan civilians have been killed in the war.

On the campaign trail and before, Trump often advocated a more isolationist approach to American foreign policy. He excoriated President George W. Bush for invading Iraq (though in a 2002 radio interview Trump indicated support for the invasion), and regularly criticized Obama for continuing to wage the war in Afghanistan.

“We should leave Afghanistan immediately,” Trump wrote on Twitter in March 2013.

“No more wasted lives. If we have to go back in, we go in hard & quick. Rebuild the US first.”

In November of that year, he tweeted: “We have wasted an enormous amount of blood and treasure in Afghanistan. Their government has zero appreciation. Let’s get out!”

Trump announces increase in military presence in Afghanistan

There are currently about 8,400 American troops deployed in Afghanistan, down from a peak of about 100,000 in 2011, when Obama ordered a huge surge in the U.S. presence there before a rapid drawdown. Most of the American troops in the country now are involved in training and advising Afghan security forces. Some are also involved in active counterterrorism operations against Islamic State militants in the country.

For Trump, whose rapid rise to the White House was built on the repeated shattering of political norms, Monday night’s address represented an acceptance of the same broad policy in the region that guided both Bush and Obama. He seemed to have accepted, grudgingly, the grim reality that American boots on the ground remain a critical ingredient to prevent a steady slide into chaos and violence.

For Obama, that lesson came in Iraq; for Trump, who takes great pride in undoing his predecessor’s work, his predecessor’s experience appeared to loom large.The Islamic State’s rise in Iraq, following the withdrawal of American forces there, factored into Trump’s decision, he said.

“A hasty withdrawal would create a vacuum that terrorists, including ISIS and al-Qaeda, would instantly fill,” Trump said. “In the end, we will win.”